Modeling Aqueous Alteration of Cm Carbonaceous Chondrites: Implications for Cronstedtite Formation by Water-rock Reaction

Introduction: CM carbonaceous chondrites display a wide range of aqueous alteration products that occurred by low temperature water-rock reactions on their parent bodies [1,2]. Previous modeling studies have verified that these reactions occurred at low temperatures (0-50C), for they reproduced many of the observed mineral assemblages [3]. The dominant phases are tochilinite and Mg-Fe phyllosilicates, specifically chrysotile, greenalite, and cronstedtite [4]. Petrographic and TEM studies on CM matrix have suggested that tochilinite and cronstedtite as ubiquitous phases, as well as possible indicators of the degree of alteration [5]. The stability field of tochilinite indicates a low f(O2), low f(S2) environment [6], but conditions of cronstedtite formation have not previously been investigated. In this study, we explored the implications of cronstedtite as the dominant Fephyllosilicate in many CM meteorites, composing up to 58% of the matrix phases along with tochilinite [7]. Conceptual Model: Previous modeling [3,8] has explored the effects of CO2 content of the fluid and initial anhydrous assemblage on the observed alteration products. This approach will be extended, investigating how the CO2 composition of the initial fluid impacts the stability of cronstedtite. Three simple, instructive initial assemblages are considered: pure olivine (50% Fo, 50% Fa); an enstatite-olivine assemblage (40% Fa, 30% Fo, 30% En); and an assemblage containing native iron metal (30% Fa, 10% iron metal, 30% Fo, 30% En). Initial anhydrous assemblages have been estimated for CM chondrites using mass balance [9]; however, here the goal is to understand the first-order effects of fluid composition and initial assemblage on the stability of cronstedtite. 1000 cm of rock were reacted with 1 kg of H2O, with starting compositions of pure water, 0.01m HCO3, and 0.1m HCO3. Both the composition of the reacted fluid and the progressive changes in mineralogy were investigated. We also considered the effect of a briny solution on the mineral assemblages and the stability of cronstedtite in particular. Oxygen isotope data, as well as aforementioned modeling studies [8,10], allow for the possibility that water flowed from the warmer interior of the parent body out to the surface. If this occurred, the water reacting with the CM anhydrous precursor would not be a pure water-CO2 fluid; rather, it would also contain mobile elements leached from the previous water-rock reactions. As a first approximation for modeling reactions with brine-like fluid, seawater was reacted with the three initial assemblages. Seawater has an HCO3 concentration of 0.002m, much of which complexes with cations. While not a likely composition for meteoritic fluids (seawater contains abundant dissolved O2 and SO4, for example), the results offer insight into other factors affecting cronstedtite stability. Computational Methods: These calculations were performed with REACT numerical code [11] using the LLNL thermodynamic database. In all calculations, chrysotile was the Mg-serpentine allowed to form, kinetically inhibited phases (i.e., quartz) were suppressed, and the phase FeO(c) was omitted due to its absence in the meteorite record. Solid solutions were not considered in these calculations. Seawater data was also taken from [11]. All calculations were performed at 25C. Results: The pure olivine (O), olivine-enstatite (OE), and olivine-enstatite-iron metal (OEI) assemblages were all reacted with the various initial fluids. In the case of pure water, none of the initial assemblages yielded cronstedtite in any substantial amount unless water-rock ratios were exceedingly high (>1000) (mass basis). Increasing HCO3 in the fluid, however, produces cronstedtite at progressively lower water-rock ratios in all starting mineral assemblages. The OE assemblage (most oxidized) altered to cronstedtite (~20-40%) over a range of moderate